High Step-Up Ratio Modular Multilevel Transformer-less DC-DC Converter for HVDC Grid of Large Scale Photovoltaic Systems

摘要

This paper presents the analysis and design of a high step-up transformerless dc-dc modular multilevel converter (MMC) for a large-scale photovoltaic power system to achieve a high voltage ratio and to interface photovoltaic power sources directly on HVDC systems. The circuit topology is a technology for high step-up dc-dc conversion ratio using the MMC approach operating in resonant mode in order to improve overall efficiency. The input voltage for this converter is a set of series photovoltaic (PV) strings. The proposed converter operates to step-up the input voltage with 1:10 conversion ratio. The output of the converter is connected to a HVDC system. Further, this type of converter has bidirectional capability and can be used as a step-down converter for microgrid applications. A detailed MATLAB/Simulink™ analysis model was developed, and indicated very fast response (less than 5 ms) in compensating for load changes and voltage fluctuations. A feedback control method is also presented. Simulation results show the advantages of using this type of converter compared to conventional methods in that it does not require a large number of isolation voltage transformers.